The present invention relates to a linear image sensor of the contact type used in an image scanner, a facsimile machine, etc.
There is known an image sensor of the contact type comprised of a plurality of image sensor chips arranged linearly with one another. FIG. 3 is a plan view of an example of such a contact type linear image sensor. A plurality of image sensor chips 21 are arranged linearly on a base plate 11. FIG. 4 shows a plan view of one image sensor chip 21. The chip 21 is provided with a linear array of picture elements 31 arranged such that an end element is spaced an interval b from an inner adjacent element and the remaining intermediate elements are spaced an interval a from each other. The end interval b is set smaller than the intermediate interval a. Further, the intermediate interval a is set exactly identical to a given standard reading pitch of the sensor in the horizontal or main scanning direction. The image sensor chip 21 is comprised of a semiconductor IC, and each picture element 31 has a light receiving region composed of a diffusion layer having one conductivity type opposite to that of a semiconductor substrate of the IC chip. The end interval b is set smaller than the intermediate interval a in order to compensate for a gap between adjacent image sensor chips 21 to maintain overall regularlity of the standard reading pitch.
If an end element 31 were disposed too closely to an edge of the chip with the regular interval a, there would be caused a drawback that leakage current would flow excessively between the light receiving diffusion layer and the semiconductor substrate.
However, the above noted conventional image sensor of the contact type has a drawback that the output level of the end picture element is varied relative to the remaining intermediate elements. Namely, since the end interval b between the end element and the inner adjacent element is set smaller than the intermediate interval a of the remaining elements, the electrode pattern layout around the end element is different from those around the remaining elements. Stated otherwise, there is caused a parasitic capacitance variation between a signal electrode pattern around an individual picture element and a light receiving region, thereby generating nonuniform outputs. It would be quite difficult to compensate for such variation provisionally by pattern design.